1. Field of the Invention
The present invention relates to a transconductor tuning device, and more particularly, to a tuning device which tunes a transconductance of a transconductor using a current.
2. Description of the Related Art
A transconductor is a circuit that outputs a current in proportion to a voltage applied to the transconductor as an input signal. The level of output current is calculated by multiplying the input voltage by a transconductance (gm). The transconductor is widely used in an integrated circuit for processing an analog signal, such as operational transconductance amplifier (OTA) and a filter. In order to realize the transconductor, a Metal Oxide Semiconductor (MOS) transistor or a bipolar transistor may be used.
However, the transconductance causes an error due to changes in power supply voltage, temperature and manufacturing process. Therefore, if the transconductor is used in a circuit requiring an accurate transconductance, the transconductor requires a transconductor tuning circuit to maintain a constant transconductance.
FIG. 1 is a block diagram illustrating a conventional transconductor tuning device. Referring to FIG. 1, the transconductor tuning device comprises a plurality of transconductors 10_1 to 10_n and a tuning circuit 20. When a current is applied to an external resistor Rext and forms a potential difference, the tuning circuit 20 outputs a certain level of control voltage Vb using the potential difference.
One example of the tuning circuit 20 is disclosed in the publication titled “A 20-MHz sixth-order BiCMOS parasitic insensitive continuous-time filter and second-order equalizer optimized for disc-driver read channels”, IEEE J, Solid-State Circuits, vol.28, pp. 462-470, April 1993.
FIG. 2 illustrates the tuning circuit 20 disclosed in the above publication. Referring to FIG. 2, the tuning circuit 20 controls transconductances of the transconductors 10_1 to 10_n according to the control voltage Vb that is output from an amplifier A3. For this, the tuning circuit 20 comprises a plurality of MOS transistors M1 to M10, a plurality of bipolar transistors Q6 and Q9, and a plurality of amplifiers A1 to A4. The amplifier A4 for generating a common-mode feedback (CMFB) voltage makes source-drain currents output from the MOS transistors M5 and M8 equal. If a current source is connected to drain terminals of the MOS transistor M5 and M8 and applies a current of kTΔi/2, the MOS transistors M7 and M10 have a difference of kTΔi in source drain currents.
Two reference voltages generated by the external resistance Rext and a reference current i are applied to gate terminals of the MOS transistors M7 and M10 and generates transconductances corresponding to currents flowing in the respective MOS transistors M7 and M10. There occurs a transconductance difference between the MOS transistors M7 and M10. In order to compensate for the transconductance difference kTΔi, the amplifier A3 generates a base voltage Vb of the bipolar transistors Q6 and Q9. If the base voltage Vb is applied to an external transconductor, the transconductor has a constant transconductance due to the kTΔi regardless of changes in characteristics of the MOS transistors.
If there are a plurality of transconductors as shown in FIG. 1, the control voltage Vb output from the tuning circuit 20 is applied to the respective transconductors 10_1 to 10_n. Even if a normal control voltage Vb is output from the tuning circuit 20, different levels of tuning voltages are applied to the respective transconductors 10_1 to 10_n depending on the respective locations of the transconductors 10_1 to 10_n due to the change in a ground reference voltage caused by a ground resistance and a ground current. More specifically, when the control voltage Vb is applied to the first transconductor 10_1, it is changed to a tuning voltage Vb1 due to a ground resistor Rg1, and a ground current flowing to the first transconductor 10_1. In the same way, the control voltage Vb is changed to a tuning voltage Vb2 when being applied to the second transconductor 10_2. As a result, different levels of tuning voltages are applied to the respective transconductors 10_1 to 10_n. To this end, the respective transconductors 10_1 to 10_n have different levels of transconductances gm1, gm2, . . . , gmn. Meanwhile, if the transconductors 10_1 to 10_n are realized by MOS, a mismatching is likely to occur among the MOSs as a layout area occupying a chip increases. Accordingly, the transconductance difference becomes greater.
In other word, it is impossible to tune the transconductors 10_1 to 10_n to have an equal transconductance.
Also, since the conventional tuning circuit 20 uses the OP amplifier A3 at an output terminal to generate a transconductance control voltage, there may occur an error due to a DC offset of the OP amplifier A3.